A common problem encountered in architectural photography that is present when taking pictures of buildings is that of converging verticals resulting in a distorted image of the building. Converging verticals occurs when the plane of the object to be photographed is not parallel with the plane of the film in the back of the camera. The problem of converging verticals is particularly acute when a photographer is using a wide angle lens and looking up at a building in an effort to capture an image of the entire building.
The prior art has devised several methods of combating the problem of vertical distortion. For example, when using a 35 mm camera, one technique for correcting vertical distortion is through the use of a tilt and shift lens when the picture is taken. As will be appreciated by one of ordinary skill in the art, a major drawback with the use of a tilt and shift lens lies in its expense; the cost of a tilt and shift lens is prohibitive for most amateur photographers. Another drawback with the use of a tilt and shift lens is that such lenses are only really suitable for use in architectural photography and difficult to use in any event.
Another common solution to avoid converging vertical distortion is the use of a view camera in which the lense is mounted on a lens board which is attached to a rear film back with a flexible bellows. Rather than tilting the camera and film back to frame a desired architectural subject, the photographer maintains the film back parallel to the plane of the architectural subject, e.g., the front of the building to be photographed. The image is then framed within the film area by translating or shifting the film back into the desired portion of the image circle projected by the lens. The flexible, light tight bellows allows independent movement of the lens board and film back to support these adjustments.
The prior art has also attempted to correct the problem of converging verticals after the photograph has been taken. Some prior artisans attempt to correct converging verticals while developing the film by tilting the enlarger's head or tilting the printing easel. These techniques suffer from several drawbacks. For example, tilting the enlarger head accurately enough to correct the convergence, or controlling the tilt of the easel, is extremely difficult in dim light when developing black and while photographs and nearly impossible in complete darkness when developing color photographs. In addition, tilting the enlarger head or easel means that one edge of the photograph paper is going to be closer than the other which may cause focusing problems. Likewise, when one edge of the paper is closer to the enlarger than the other, that edge receives more light and may become over exposed.
Other artisans have attempted to correct a photograph's perspective using digital techniques. A variety of software packages which have some sort of “distortion” tool which can map a rectangular area to an arbitrary quadrilateral may be used to attempt to correct converging verticals. A description of digital correction using a variety of software packages is set forth at http://photocritique.net/articles/converging/converging.html (visited Oct. 26, 2000). Drawbacks of using commercial software packages to perform digital compression are numerous. For example, using commercial software requires a user to have a computer system capable of running the software and requires the photographs to be fully developed and then input into the computer system by means of a scanner which provides the system with a digital representation of the photograph. These components can be prohibitively expensive. In addition, attempting to digitally correct for converging verticals after the photograph has been developed and scanned into a computer system for editing (and then saving the corrected image) can result in loss of detail. Alternatively, while digital cameras may provide a digitized image directly, such images are often stored using a lossy compression algorithm so that later image manipulation produces undesirable image artifacts.
The foregoing underscores some of the problems and limitations associated with correcting converging verticals in optical images.